Detecting and Characterizing Young Quasars. II. Four Quasars at z ∼ 6 with Lifetimes < 104 Yr

The extents of proximity zones of high-redshift quasars enable constraints on the timescales of quasar activity, which are fundamental for understanding the growth of the supermassive black holes (SMBHs) that power the quasars' emission. In this study, we obtain precise estimates for the ultraviolet (UV) luminous lifetimes of 10 quasars at 5.8 < z < 6.5. These objects were preselected to have short lifetimes based on preliminary measurements of their proximity zone sizes and were then targeted for high-quality follow-up submillimeter, optical, and infrared observations required to increase the measurements' precision and securely determine their lifetimes. By comparing these proximity zone sizes to mock quasar spectra generated from radiative transfer simulations at a range of different lifetimes, we deduce extremely short lifetimes t (Q) < 10(4) yr for four objects in our sample, whereas the remaining quasars are consistent with longer lifetimes of t (Q) greater than or similar to 10(5) yr. These young objects with small proximity zones represent less than or similar to 10% of the quasar population as a whole. We compare our results in detail to other studies on timescales of quasar activity, which point toward an average lifetime of t (Q) similar to 10(6) yr for the quasar population. This is consistent with finding newly turned-on quasars approximately similar to 1%-10% of the time. These young quasars represent a unique opportunity to study triggering and feedback mechanisms of SMBHs, since the onset of their UV-luminous quasar phase happened only recently, and therefore traces of this process might still be observable.